Impact of climate, vegetation, soil and crop management variables on multi-year ISBA-A-gs simulations of evapotranspiration over a Mediterranean crop site

Abstract

Abstract. Generic land surface models are generally driven by large-scale data sets to describe the climate, the soil properties, the vegetation dynamic and the cropland management (irrigation). This paper investigates the uncertainties in these drivers and their impacts on the evapotranspiration (ET) simulated from the Interactions between Soil, Biosphere, and Atmosphere (ISBA-A-gs) land surface model over a 12-year Mediterranean crop succession. We evaluate the forcing data sets used in the standard implementation of ISBA over France where the model is driven by the SAFRAN (Système d'Analyse Fournissant des Renseignements Adaptés à la Nivologie) high spatial resolution atmospheric reanalysis, the leaf area index (LAI) time courses derived from the ECOCLIMAP-II land surface parameter database and the soil texture derived from the French soil database. For climate, we focus on the radiations and rainfall variables and we test additional data sets which include the ERA-Interim (ERA-I) low spatial resolution reanalysis, the Global Precipitation Climatology Centre data set (GPCC) and the MeteoSat Second Generation (MSG) satellite estimate of downwelling shortwave radiations.The evaluation of the drivers indicates very low bias in daily downwelling shortwave radiation for ERA-I (2.5 W m −2 ) compared to the negative biases found for SAFRAN (−10 W m −2 ) and the MSG satellite (−12 W m −2 ). Both SAFRAN and ERA-I underestimate downwelling longwave radiations by −12 and −16 W m −2 , respectively. The SAFRAN and ERA-I/GPCC rainfall are slightly biased at daily and longer timescales (1 and 0.5 % of the mean rainfall measurement). The SAFRAN rainfall is more precise than the ERA-I/GPCC estimate which shows larger inter-annual variability in yearly rainfall error (up to 100 mm). The ECOCLIMAP-II LAI climatology does not properly resolve Mediterranean crop phenology and underestimates the bare soil period which leads to an overall overestimation of LAI over the crop succession. The simulation of irrigation by the model provides an accurate irrigation amount over the crop cycle but the timing of irrigation occurrences is frequently unrealistic.Errors in the soil hydrodynamic parameters and the lack of irrigation in the simulation have the largest influence on ET compared to uncertainties in the large-scale climate reanalysis and the LAI climatology. Among climate variables, the errors in yearly ET are mainly related to the errors in yearly rainfall. The underestimation of the available water capacity and the soil hydraulic diffusivity induce a large underestimation of ET over 12 years. The underestimation of radiations by the reanalyses and the absence of irrigation in the simulation lead to the underestimation of ET while the overall overestimation of LAI by the ECOCLIMAP-II climatology induces an overestimation of ET over 12 years.This work shows that the key challenges to monitor the water balance of cropland at regional scale concern the representation of the spatial distribution of the soil hydrodynamic Published by Cop...

Effects of high spatial and temporal resolution Earth observations on simulated hydrometeorological variables in a cropland (southwestern France)

“…A gravitational drainage flux is simulated when the soil water content of a layer exceeds the field capacity. In the version we used, a subgrid runoff is also calculated using the variable infiltration capacity scheme first described by Dumenil and Todini (1992) and included in SURFEX by Habets et al (1999). It allows simulating a runoff flux even when the soil is not fully saturated.…”

Hydrological assessment of atmospheric forcing uncertainty in the Euro-Mediterranean area using a land surface model

“…1. coupling between above-ground biomass dynamics and soil water balance ("NIT") (Calvet and Soussana, 2001;Gibelin et al, 2006); 2. multilayer soil diffusion scheme explicitly solving the soil water and heat balance equations ("DF") (Boone et al, 2000;Decharme et al, 2011Decharme et al, , 2013.…”

“…Smith et al, 2010a, b). Similarly, irrigation modelling and its impacts on sim-ulated agricultural production need to be validated at large spatial scales: Garrigues et al (2015) evaluated how ISBA-A-gs simulated evapotranspiration over an irrigated Mediterranean agricultural site. Dynamic vegetation modelling and realistic representations of irrigation are crucial to predict crop yield and assess the sustainability of crop water management (Jägermeyr et al, 2016).…”

“…We use the ISBA-A-gs LSM (Noilhan and Planton, 1989;Calvet et al, 1998) coupled with the CTRIP (standing for CNRM TRIP) river routing model (Decharme et al, 2010), an upgraded version of the TRIP model (Oki and Sud, 1997), to simulate the energy and water balances of the land surface as well as plant phenological development. ISBA-A-gs is a CO 2 -responsive LSM that simulates vegetation dynamics and provides prognostic estimates of LAI (in the configuration described by Gibelin et al, 2006). It also features a multi-layer diffusion scheme for the soil water and energy balances (Decharme et al, 2013).…”

Response to Reviewer 1

“…1. coupling between above-ground biomass dynamics and soil water balance ("NIT") (Calvet and Soussana, 2001;Gibelin et al, 2006); 2. multilayer soil diffusion scheme explicitly solving the soil water and heat balance equations ("DF") (Boone et al, 2000;Decharme et al, 2011Decharme et al, , 2013.…”